This invention relates to fuel cell construction and, in particular, to a method of preparing a catalyst plate for use in, in situ, reforming of process fuels such as hydrocarbons and alcohols. This invention also relates to fuel cells in which in situ or internal reforming is carried out utilizing reforming catalysts.
It has been recognized that in fuel cell operation, particularly, high temperature fuel cell operation such as found in molten carbonate and solid oxide cells, the heat generated can be used to reform the hydrocarbon content of the fuel cell process gas. The hydrocarbon content of fuel cell process gas frequently contains methane and other hydrocarbons such as, for example, propane, methanol, ethanol and other reformable organic fuels, and as used herein is also intended to include alcohols. The heat value on a mole basis, and, hence, electrical energy producing potential of methane is about three to four times greater than that of hydrogen. Since methane itself is relatively electrochemically inactive, it is very desirable to reform methane to form hydrogen and carbon monoxide in accordance with the reaction: CH.sub.4 +H.sub.2 O.fwdarw.3H.sub.2 +CO. The hydrogen and carbon monoxide can then participate in the fuel cell reaction either directly or by a further water-gas shift. An incentive for carrying out such reforming reaction in a fuel cell is that the reaction is endothermic and would serve to offset heat generated in fuel cell operation due to inherent irreversibility.
U.S. Pat. No. 3,488,226 discloses a fuel cell construction wherein reforming of process gas hydrocarbons is carried out in situ by placement of a suitable catalyst in direct heat exchange relationship to the cell. This patent teaches that placement of the catalyst uniformly along the length of the cell results in a reduction in the maximum temperature of the cell. It also mentions that by locating catalyst in the vicinity of the centroid of the cell a further reduction in the maximum temperature can be achieved.
In the '226 patent, the catalyst is in the form of nickel alumina-aluminum pellets of approximately one-half inch in diameter. These pellets are produced by crushing a nickel aluminum alloy and treating the resulting particles with a sodium hydroxide solution. The resultant mixture is maintained at its boiling point and allowed to undergo conversion of the aluminum to sodium aluminate and alumina. After the desired conversion, the reaction is quenched with water. Subsequent washings with water are followed by washings with methanol and the resultant pellets, thereafter, are stored in methanol.
U.S. Pat. No. 4,182,795, assigned to the same assigned hereof, discloses an improved construction wherein in situ hydrocarbon reforming is via a catalyst placed in an electrolyte-isolated passage, this passage being in heat transfer relationship with the cell. Such placement of the catalyst prevents electrolyte condensation which would normally occur in an electrolyte-communicative passage at cold spots created by the endothermic reforming reaction. The process gas in the electrolyte-isolated passage also acts as a cooling means so that cooling of the cell and reforming are simultaneously brought about by the single passage.
Disposition of the catalyst in the '795 patent construction is in layered or packed form uniformly along and on a plate defining the electrolyte-isolated passage. The configuration of the catalyst coated plate is U-shaped or corrugated with the catalyst being placed on the upper plate walls.
Finally, the '795 patent also mentions that a suitable catalyst for reforming methane hydrocarbon content is nickel or nickel based and that a commercially available version of such catalyst is Girdler G-56 which is provided in pellet form for packing in fixed bed type reactors.
Other practices, not specifically directed to in situ reforming in a fuel cell, but directed to forming catalyst members for hydrocarbon reforming in other applications are also known. U.S. Pat. No. 4,019,969 teaches a method for manufacturing catalytic tubes in which a metallic sponge is formed on the inner wall of a metallic tube by electrolysis. The sponge is then impregnated with appropriate salts of catalytic and ceramic substances and the assembly then roasted to provide the desired catalytic member.
U.S. Pat. No. 3,186,957 teaches a technique for forming pellet catalysts in which a slurry of alpha alumina hydrate and a soluble nickel salt are coprecipitated and, thereafter, the product calcinated at a low temperature to produce nickel oxide supported on a ceramic oxide (alumina). The coprecipitate is then formed into suitable pellet shapes and heated at a high temperature to establish a nickel aluminate interface between the nickel oxide and the ceramic oxide.
In U.S. Pat. No. 3,498,927 the starting material is a refractory oxide material which is gelled and to which is added, before or after gelling, a catalytic metal. The gel of the catalytic metal supported on the refractory material is then applied to a ceramic support structure, either by spraying or immersing. The product is then dried and calcinated to form the resultant catalyst.
U.S. Pat. No. 3,645,915 discloses a technique in which a catalyst comprised of nickel oxide, nickel chromite and a stabilizer are placed in a slurry form and the slurry applied to a refractory oxide or metallic support by impregnation or cementing. The resultant product is then calcined. When the support is metallic, the support may be roughened to provide an anchor for the applied materials.
U.S. Pat. No. 3,513,109 discloses use of a slurry of catalytic material and metal ammines and application of same to a refractory support. The slurry also may be provided with a refractory interspersant prior to applying the slurry to support. Such application may be by spraying or dipping and is followed by drying and subsequent calcination.
U.S. Pat. No. 3,627,790 teaches formation of a Raney nickel (Ni-Al.sub.3) type catalyst by partially leaching aluminum from a nickel-aluminum alloy. This type catalyst is to be used for hydrogeneration at the fuel cell anode and not for reforming. A further U.S. Pat. No. 4,024,075, discloses a cobalt based catalyst for low temperature operation without significant carbon deposition.
While the above patents and practices for making catalysts have proved useful in the formation of certain forms of catalyst members, i.e., pellets, honeycombs, tubular structures, further practices are still being investigated as regards formation of such members to meet the stringent requirements of in situ fuel cell reforming. In such reforming the following conditions must be satisfied: (a) the catalyst must adhere to a metallic plate having an extended continuous surface; (b) the catalyst must be able to provide satisfactory reforming rates in the range of 1000.degree. F. to 1300.degree. F. and 1-10 atm operating pressure; (c) the catalyst must be stable in the presence of fuel cell electrolyte and at cell operating temperatures; (d) the catalyst should permit operation at low steam-to-carbon (s/c) ratios; (e) the catalyst should provide long term operation before regeneration is required, since regeneration may affect cell anode stability; (f) the catalyst should provide low ohmic resistance; (g) the catalyst should have crushing strength sufficient to withstand cell sealing pressures; and (h) the catalyst should enable reasonable heat exchange.
It is also noted that these patents teach that distribution of the catalyst uniformly along the length of the cell results in a reduction of the temperature gradient in the cell and that by placing the catalyst at the centroid of the cell a further reduction in maximum temperature can be achieved. Such placement of catalyst taught by these patents however is not believed to provide maximum performance for the cell and may, in fact, adversely affect cell performance. For example, excessive cooling due to fresh fuel reforming at the inlet might cause freezing of the cell electrolyte in a molten carbonate fuel cell, while in a solid oxide cell, the conductivity of the electrolyte might be greatly reduced. Furthermore, the disclosed placements are not believed to promote uniform current density and/or uniform temperature distribution in the cell.
It is an object of the present invention to provide an improved fuel cell catalyst member and practice for in situ reforming of process fuels.
It is a further object of the present invention to provide a practice for realizing a fuel cell catalyst member meeting the above-mentioned requirements.
It is a further object of the present invention to provide a catalyst member of the aforesaid type which is adaptable for use in molten carbonate and solid oxide fuel cells.
It is a further object of the present invention to provide a fuel cell having a catalyst therein for, in situ, or internal reforming and which catalyst is adapted to provide improved fuel cell performance.
It is still a further object of the present invention to provide a fuel cell of the above-mentioned type wherein the catalyst is adapted to promote uniform temperature distribution and/or uniform current density in the cell.